1. Field of the Invention
In general, the systems and methods of this invention relate to the determination of transmission line characteristics. In particular, this invention relates to systems and methods for determining the characteristics of a transmission line using broadband signals.
2. Description of Related Art
Rapid developments in the computer industry and the availability of affordable hardware created the Internet, i.e., a distributed network, wherein a user having a communications link between themselves and a computer in a centralized location can access publicly available information. Users of the Internet are connected to the distributed network through a link that includes, for example, a telephone line from a customer premises (CPE) to a telephone company central office (CO). A user requesting a data transfer from an Internet server is faced with the limited bandwidth of the connection between their location and the central office. As more and more information is being created and stored in digital format, the demand for users to access large data files is increasingly making it crucial to find new and faster ways of transferring data. One way of achieving faster data transmission is to increase the bandwidth of the transmission line between the users and the CO by, for example, replacing the current metallic conductors with fiber or using better quality metallic conductors having increased bandwidth. However, such an approach is costly and requires a substantial investment by the telephone companies.
Recent developments in digital signal processing and telecommunications have resulted in the digital subscriber line (DSL) technology enabling a high speed data link over existing twisted pair telephone lines. Although a couple of different DSL systems have been proposed, multi-carrier systems have quickly gained popularity and are becoming standardized. Multi-carrier DSL systems operate on the principle of frequency division multiplexing, wherein separate frequency bands are used to transfer data from the CPE to the CO and vice versa. The portion of the bandwidth allocated for transmitting data from the user to the CO is called the upstream (US) channel, and the portion of bandwidth allocated for passing data from the CO to the user is called the downstream (DS) channel. Since in a typical Internet session the amount of data being transferred from the CO to the user is much larger than the amount of data transmitted from the user to the CO, the bandwidth allocated for the downstream channel is usually much larger than the bandwidth allocated for the upstream channel. Typical ratios of downstream to upstream channel bandwidth are 4:1 or 8:1.
The bandwidth allocated to the upstream and downstream channels is partitioned into a large number of sub-bands which are sufficiently narrow so as to allow the distortions introduced by the line to be described as an attenuation and a phase shift. These parameters can be measured in a training session prior to establishing the data link by sending and receiving a predefined signal on a sub-band. The amount of data that can be sent in a sub-band is limited by the signal to noise ratio (SNR) in that sub-band, which is the signal strength described by the line attenuation divided by the noise power. Each of the sub-bands in the multi-carrier DSL system is used to transmit data that is consistent with the SNR on that sub-band and maximum allowable bit error rate (BER). A multi-carrier DSL system operating within the principles described above is able to achieve data rates that are as high as, for example, ten million bits per second.